The five modes of heat generation in a Li-ion cell under discharge

A lithium-ion cell under discharge generates thermal energy (Q) through five different internal parameters or modes: the electrolyte resistance (R-s), anode resistance (R-a), cathode resistance (R-c), and entropy changes in the cathode (Delta S-c), and the anode (Delta S-a). This work demonstrates a set of tools to measure/quantify the heat generated by each parameter separately during discharge. These five sources are not dependent upon each other; they are dependent on the state of charge and the environmental temperature (T-env). The Q generated by each mode varies with degree of discharge and T-env. R-s generates most of the Q in the -10 degrees C to 40 degrees C range; R-c becomes significant at T-env <20 degrees C. Constant current discharge does not cause a monotonic increase in anode and cathode temperatures (T-a and T-c), due to the direction of change in Delta S-c and Delta S-a. Negative change in Delta S-a for the carbon anode cools it, causing the T-a to level off and even decrease with increased discharge. Delta S-c for lithium manganese oxide cathode is positive at some SoC and negative at others, preventing a monotonic increase in T-c. Measuring the five Qs separately opens the opportunity to study thermal-runaway from the perspective of the anode, cathode and electrolyte. (C) 2014 Elsevier B.V. All rights reserved.